It has been known that the moisture in the keratinous layer plays an important role of moistening and softening the skin and that water soluble components of the keratinous layer (i.e., free amino acids, urea, organic acids, inorganic ions, etc.) participate in the retention of the moisture. Accordingly, these water soluble substances are used in dermatologic preparations and cosmetics in order to ameliorate or prevent skin chapping.
For the same purposes, there have been developed and employed various moisturizing substances having strong affinities for water. When applied to the skin, however, such a moisturizing substance remains on the keratinous layer of the skin and supplies moisture thereto. Moreover, it can exert only a temporary effect. Thus it can neither fundamentally improve the moisture retention of the keratinous layer nor essentially prevent or treat skin chapping.
On the other hand, it is known that ceramides, which are contained in the keratinous layer of the skin, contribute to the barrier function and moisture retention of the skin. It is also known that the external use of ceramides is efficacious in treating dry skin. Therefore, attempts have been made to synthesize ceramides or analogues of the same (i.e., pseudoceramides). For example, methods for producing ceramides per se are described in JP-A-59-7118 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and WO 93/22281, while methods for producing pseudoceramides are described in, for example, JP-A-62-228048, JP-A-63-22107, JP-A-63-216812, JP-A-63-218609, JP-A-63-227513, JP-A-63-227514, JP-A-63-228048, JP-A-63-297309, JP-A-64-9906, JP-A-64-9907, JP-A-64-29347, JP-A-64-31752, JP-A-64-79195, JP-A-4-225907, JP-A-4-282304, JP-A-4-342553, EP No. 554303 and EP No. 555250.
Recently, it has been also proposed to use ceramides and/or pseudoceramides together with hydroxycarboxylic acids such as glycolic acid or lactic acid to thereby efficaciously ameliorate dry skin (EP No. 587288 corresponding to JP-A-6-157283). However, it is known that hydroxycarboxylic acids would irritate the skin when applied to a sensitive part such as the face. It is therefore difficult to use a hydrocarboxylic acid in such an amount as to give a satisfactory effect as a component of a face cosmetic.